Reducing power consumption is always of the essence to ubiquitous wireless communication. For a low activity rate system, the natural method to take this advantage is duty-cycling turning on and off the device to reduce average power consumption so as to increase the battery life time by orders of magnitude. Following this trend, an always-on wake-up receiver (WuRx) 100, as shown in FIG. 1, becomes a firm demand used to continuously monitor the radio link for communication requests, e.g. between an antenna 102 and a data receiver 104, to power on the receiver 104 which is in deep sleep mode. As shown by the timing diagram 106, the wake-up receiver (WuRx) 100 continuously monitors the radio link or channel for communication requests for the main receiver 104, except during the period when there are communication requests for transmitting and receiving data, as shown by the timing diagrams 108, 110.
This auxiliary receiver, in the form of the wake-up receiver (WuRx) 100, breaks the trade-off between latency and average power consumption with only one receiver existing. Because the WuRx 100 is continuously monitoring the channel, its active power consumption must be very low. For those transceiver architectures that offer high efficiency communication but suffer from long synchronization time, such as radio-frequency identification (RFID), ultra-wideband (UWB) and electronic toll collection (ETC), WuRx are good candidates for wake-up based synchronization.
The early days' receiver was very simple, such as AM receivers, and can be implemented with an antenna, radio-frequency (RF) amplification and a nonlinear envelope detector to demodulate the signal. There are no power hungry local oscillators and mixers, which are found on the RF signal path, altogether in the frequency conversion counterpart nowadays. The envelope detector, usually implemented with a diode, is a popular choice because of its low power consumption. Envelope detectors have the inherent disadvantage of their quadratic nonlinearity that means a factor two drop in efficiency. Each drop of 10 dB in the input RF amplitude will result in a drop of 20 dB in the demodulated amplitude. This nature limits the sensitivity of the detector and of the overall receiver, owing to the signal dependent gain of the envelope detector. Indeed, the detector is the bottleneck of the receiver's sensitivity since it attenuates low level input signal and adds excessive noise. Not only high gain amplification for sensitivity but also narrowband filtering at RF for selectivity are required to overcome this limitation. However, the addition of RF gain stage is expensive from a power perspective. For example, more than 80% of the total receiver power is consumed at the gain stages, by the low noise amplifier (LNA), followed by the antenna and the channel-select amplifiers. The power breakdown illustrates the critical problem that large amounts of power are required at the RF gain stage. This prohibits further reduction in power consumption if the gain stage at higher carrier frequency is mandatory in certain applications.
There is another simple and low power wireless receiver candidate, a passive RFID tag, which does not even have a power supply. The RFID tag is inactive until it rectifies the remote RF energy from the reader to power up its own electronics and then to decode an incoming signal. In this way, the operation of the tag is very similar to the desired functionality of the WuRx. However, the rectifier has several drawbacks. The two main loss factors are from the threshold voltage of MOS diode and input parasitic capacitance of the rectifier. Hence, the communication range for the passive RFID tag is short, usually in the range of couples of centimeters, and the sensitivity is limited, at about −25.7 dBm on a 300 ohms antenna in the 2.4 GHz band. Although the RFID tag receiver features attractively low power consumption, a practical WuRx design will require much improvement to overcome the above addressed sensitivity and selectivity limitation.
Therefore, simple RFID receivers design does not satisfy the requirement for WuRx due to its low sensitivity, while conventional frequency conversion architectures are inherently too complicated and limited by power consumption. Clearly, the feasibility of implementing a WuRx receiver at high frequencies with low power dissipation represents a significant challenge.